Simple Dip-Coating Process for the Synthesis of Small Diameter Single-Walled Carbon Nanotubes-Effect of Catalyst Composition and Catalyst Particle Size on Chirality and Diameter

We report on a dip-coating method to prepare catalyst particles (mixture of iron and cobalt) with a controlled diameter distribution on silicon wafer substrates by changing the solution's concentration and withdrawal velocity. The size and distribution of the prepared catalyst particles were analyzed by atomic force microscopy. Carbon nanotubes were grown by chemical vapor deposition on the substrates with the prepared catalyst particles. By decreasing the catalyst particle size to below 10 nm, the growth of carbon nanotubes can be tuned from few-walled carbon nanotubes, with homogeneous diameter, to highly pure single-walled carbon nanotubes. Analysis of the Raman radial breathing modes, using three different Raman excitation wavelengths (488, 633, and 785 nm), showed a relatively broad diameter distribution (0.8-1.4 nm) of single-walled carbon nanotubes with different chiralities. However, by changing the composition of the catalyst particles while maintaining the growth parameters, the chiralities of single-walled carbon nanotubes were reduced to mainly four different types, (12, 1), (12, 0), (8, 5), and (7, 5), accounting for about 70% of all nanotubes.     

Dispersion of single-walled carbon nanotubes in dimethylacetamide and a dimethylacetamide–cholic acid mixture

A way of dispersing single-walled carbon nanotubes in preparing stable suspensions with high concentrations of individual nanotubes in amide solvents is described. The obtained suspensions are studied via Raman spectroscopy. The dependence of the degree of single-walled carbon nanotube (SWNT) dispersion in individual and mixed amide solvents on the type of solvent, the mass of nanotubes, and the concentration of cholic acid is established. A technique for processing spectral data to estimate the diameters and chiralities of individual nanotubes in suspension is described in detail.     

Changes in single-walled carbon nanotube chirality during growth and regrowth

A simple model for joining two single-walled carbon nanotubes (SWNTs) with different, arbitrary chiralities is used to systematically label junction structures which contain pentagon-heptagon pairs. The model is also used, together with density functional theory, to study the energetics of diameter and chirality changes of thin SWNTs during catalyzed growth or regrowth. We choose zigzag and armchair SWNTs attached to a Ni(55) cluster for our case studies.     

Dendron-tethered and templated CdS quantum dots on single-walled carbon nanotubes

CdS nanoparticles on the surface of single-walled carbon nanotubes (SWNTs) were templated and stabilized through the initial attachment of 1 --> 3 C-branched amide-based dendrons and were both photophysically and morphologically characterized. The CdS clusters were shown to be ca. 1.4 nm in diameter as calculated from their optical absorption spectra and exhibited reduced fluorescence emission intensity at 434 nm compared to that of CdS quantum dots stabilized by untethered dendrons due to partial emission quenching by the SWNT. Unchanged UV absorption behavior of these materials indicated that they are stable > 90 days at 25 degrees C.     

Electrochemical doping of chirality-resolved carbon nanotubes

Raman spectra of electrochemically charged single-wall carbon nanotubes (HiPco) were studied by five different laser photon energies between 1.56 and 1.92 eV. The bands of radial breathing modes (RBM) were assigned to defined chiralities by using the experimental Kataura plot. The particular (n,m) tubes exhibit different sensitivity to electrochemical doping, monitored as the attenuation of the RBM intensities. Tubes which are in good resonance with the exciting laser exhibit strong doping-induced drop of the RBM intensity. On the other hand, tubes whose optical transition energy is larger than the energy of an exciting photon show only small changes of their RBM intensities upon doping. This rule presents a tool for analysis of mixtures of single-walled carbon tubes of unknown chiralities. It also asks for a re-interpretation of some earlier results which were reported on the diameter-selectivity of doping. The radial breathing mode in strongly n- or p-doped nanotubes exhibited a blue-shift. A suggested interpretation follows from the charging-induced structural changes of SWCNTs bundles, which also includes a partial de-bundling of tube ropes.     

Aqueous dispersion, surface thiolation, and direct self-assembly of carbon nanotubes on gold

We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces water-soluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.     

不同结构碳纳米管的电磁波吸收性能研究

研究了单壁、多壁碳纳米管(聚团状、阵列状)以及未纯化与纯化后碳纳米管在2～18 GHz范围内的电磁波吸收性能. 通过测定不同结构碳纳米管粉体的介电常数以及磁导率, 得到损耗因子及衰减常数大小顺序为: 阵列状多壁碳纳米管>原生聚团状多壁碳纳米管>纯化聚团状多壁碳纳米管>原生单壁碳纳米管>纯化后单壁碳纳米管. 相比多壁碳纳米管, 单壁碳纳米管衰减常数随频率变化较小, 且具有较宽的吸收峰. 模拟计算和实验测试结果都表明, 碳纳米管/聚合物复合材料具有优良的电磁吸波性能.     

Energy gaps, electronic structures, and x-ray spectroscopies of finite semiconductor single-walled carbon nanotubes

We report hybrid density functional theory calculations for electronic structures of hydrogen-terminated finite single-walled carbon nanotubes (6,5) and (8,3) up to 100 nm in length. Gap states that are mainly arisen from the hydrogen-terminated edges have been found in (8,3) tubes, but their contributions to the density of states become invisible when the tube is longer than 10 nm. The electronic structures of (6,5) and (8,3) tubes are found to be converged around 20 nm. The calculated band-gap energies of 100 nm long nanotubes are in good agreement with experimental results. The valence band structures of (6,5), (8,3), as well as (5,5) tubes are also investigated by means of ultraviolet photoelectron spectra (UPS), x-ray emission spectroscopy (XES), and the resonant inelastic x-ray scattering (RIXS) spectra theoretically. The UPS, XES and RIXS spectra become converged already at 10 nm. The length-dependent oscillation behavior is found in the RIXS spectra of (5,5) tubes, indicating that the RIXS spectra may be used to determine the size and length of metallic nanotubes. Furthermore, the chiral dependence observed in the simulated RIXS spectra suggests that RIXS spectra could be a useful technique for the determination of chirality of carbon nanotubes.     

Excitons in semiconducting carbon nanotubes: diameter-dependent photoluminescence spectra

Semiconducting single-walled carbon nanotubes are one-dimensional (1D) quantum nanostructures and their unique optical responses arise from stable 1D excitons with huge binding energies. Here we review recent studies on optical properties of semiconducting carbon nanotubes. The diameter dependence of luminescence spectra and dynamics are revealed by single-nanotube spectroscopy and time-resolved optical spectroscopy. Short-range Coulomb interactions play a crucial role in energy structures of dark, triplet, and charged excitons. Enhanced exciton-exciton interactions in 1D semiconductor nanostructures determine nonlinear optical responses. We present generic configurations of neutral and charged excitons and discuss exciton optics of single-walled carbon nanotubes.     

Molecular dynamics simulations on the effects of diameter and chirality on hydrogen adsorption in single walled carbon nanotubes

We present systematic molecular dynamics simulation studies of hydrogen storage in single walled carbon nanotubes of various diameters and chiralities using a recently developed curvature-dependent force field. Our main objective is to address the following fundamental issues: 1. For a given H2 loading and nanotube type, what is the H2 distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal loading (H2 coverage)? 3. What is the diameter range and chirality for which H2 adsorption is most energetically favorable? Our simulation results suggest strong dependence of H2 adsorption energies on the nanotube diameter but less dependence on the chirality. Substantial lattice expansion upon H2 adsorption was found. The average adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with higher H2 loading. The calculated H2 vibrational power spectra and radial distribution functions indicate a strong attractive interaction between H2 and nanotube walls. The calculated diffusion coefficients are much higher than what has been reported for H2 in microporous materials such as zeolites, indicating that diffusivity does not present a problem for hydrogen storage in carbon nanotubes.     

Constrained iron catalysts for single-walled carbon nanotube growth

The diameter of single walled carbon nanotubes (SWNTs) determines the electronic properties of the nanotube. The diameter of carbon nanotubes is dictated by the diameter of the catalyst particle. Here we describe the use of iron nanoparticles synthesized within the Dps protein cage as catalysts for the growth of single-walled carbon nanotubes. The discrete iron particles synthesized within the Dps protein cages when used as catalyst particles gives rise to single-walled carbon nanotubes with a limited diameter distribution.     

NEXAFS光谱分析聚合物修饰的单壁碳纳米管

利用C1s、O1s、N1s近边X射线吸收精细结构(Near Edge X-Ray Absorption Fine Structure,NEXAFS)光谱对聚合物修饰的碳纳米管进行了分析,研究了氧化及偶联聚合物对碳纳米管结构的影响。氧化碳纳米管及十八胺修饰的、聚合物/十八胺双修饰的碳纳米管的NEXAFS光谱均出现了碳/氧K边π*(C=O)和σ*(C-O)共振峰;而十八胺修饰的、聚合物/十八胺双修饰的碳纳米管则出现了氮K边π*(N-C=O)和σ*(N1s)共振峰。分析表明,NEXAFS光谱可有效表征聚合物修饰的碳纳米管。     

Cryogenic separation of hydrogen isotopes in single-walled carbon and boron-nitride nanotubes: insight into the mechanism of equilibrium quantum sieving in quasi-one-dimensional pores

Quasi-one-dimensional cylindrical pores of single-walled boron nitride and carbon nanotubes efficiently differentiate adsorbed hydrogen isotopes at 33 K. Extensive path integral Monte Carlo simulations revealed that the mechanisms of quantum sieving for both types of nanotubes are quantitatively similar; however, the stronger and heterogeneous external solid-fluid potential generated from single-walled boron nitride nanotubes enhanced the selectivity of deuterium over hydrogen both at zero coverage and at finite pressures. We showed that this enhancement of the D(2)/H(2) equilibrium selectivity results from larger localization of hydrogen isotopes in the interior space of single-walled boron nitride nanotubes in comparison to that of equivalent single-walled carbon nanotubes. The operating pressures for efficient quantum sieving of hydrogen isotopes are strongly depending on both the type as well as the size of the nanotube. For all investigated nanotubes, we predicted the occurrence of the minima of the D(2)/H(2) equilibrium selectivity at finite pressure. Moreover, we showed that those well-defined minima are gradually shifted upon increasing of the nanotube pore diameter. We related the nonmonotonic shape of the D(2)/H(2) equilibrium selectivity at finite pressures to the variation of the difference between the average kinetic energy computed from single-component adsorption isotherms of H(2) and D(2). In the interior space of both kinds of nanotubes hydrogen isotopes formed solid-like structures (plastic crystals) at 33 K and 10 Pa with densities above the compressed bulk para-hydrogen at 30 K and 30 MPa.     

Stabilization effect of single-walled carbon nanotubes on the functioning of photosynthetic reaction centers

The interaction between single-walled carbon nanotubes and photosynthetic reaction centers purified from purple bacterium Rhodobacter sphaeroides R-26 has been investigated. Atomic force microscopy studies provide evidence that reaction center protein can be attached effectively to the nanotubes. The typical diameter of the nanotube is 1-4 nm and 15 +/- 2 nm without and with the reaction centers, respectively. Light-induced absorption change measurements indicate the stabilization of the P+(Q(A)Q(B))- charge pair, which is formed after single saturating light excitation after the attachment to nanotubes. The separation of light-induced charges is followed by slow reorganization of the protein structure. The stabilization effect of light-initiated charges by the carbon nanotubes opens a possible direction of several applications, the most promising being in energy conversion and storage devices.     

Super-long continuous Ni nanowires encapsulated in carbon nanotubes

Super-long continuous Ni-filled carbon nanotubes were synthesized by the chemical vapor deposition (CVD) method with cloth-like single-walled carbon nanotube (SWNT) raw soot produced by the arc-discharge method as catalyst; the Ni nanowires inside the carbon nanotubes are single crystals, with an average diameter of 40 nm and up to tens of micrometres in length.     

扶手椅型单壁碳纳米管的结构和稳定性

用AM1和PM3两种方法对相同长度不同直径以及相同直径不同长度的扶手椅型单壁碳纳米管进行了优化, 并对AM1方法优化所得到的碳纳米管的几何结构进行了振动频率分析和p轨道轴矢量(POAV)分析, 发现了其热力学稳定性随碳纳米管直径和长度的变化规律, 讨论了各碳纳米管的张力、碳原子的杂化状态和共轭程度随碳纳米管直径的变化关系, 并与苯和C60做了比较。     

椅型碳纳米管吸附氧原子的密度泛函理论研究

利用密度泛函B3LYP对有限长扶手椅形单壁碳纳米管(3,3),(4,4)和(5,5)吸附O原子的几何结构、电子属性、反应能和红外光谱进行了系统地理论研究,获得了一些有意义的结果,主要包括如下4个方面:(1)2个O原子吸附在管外壁垂直于管轴的C—C键形成开环的轮烯结构,吸附在管内壁形成环氧结构;(2)O原子吸附在管外壁要比吸附在管内壁具有较大的能隙和吸附反应能;(3)与单壁碳纳米管管外壁吸附1个O原子相比,2个O原子吸附在管外壁具有较大的吸附反应能;(4)B3LYP得到的C—O伸缩振动频率与实验一致.     

Adsorption properties of SCN? on (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes: a density functional study

Abstract  

The behavior of the thiocyanate anion (SCN⁻) adsorbed on the external surface of H-capped (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes was studied by using density functional calculations. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 suite of programs. We present the nature of the SCN⁻ interaction in selected sites of the nanotubes. Our results show that the pristine carbon nanotubes cannot significantly detect SCN⁻. The calculated binding energy of the Al-doped (6,0) single-walled carbon nanotubes indicated that SCN⁻ can be adsorbed significantly on the C and Al sites and these nanotubes can therefore be used for SCN⁻ storage. Binding energies corresponding to adsorption of SCN⁻ on the Al site in the Al-doped (6,0) single-walled carbon nanotubes was calculated as −286.38 kJ mol⁻¹. The calculated binding energies for SCN⁻ in N-down orientation are higher than those in S-down orientation for all of the configurations. More efficient binding could not be achieved by increasing the nanotube diameter. We also report the effects of SCN⁻ adsorption on the electronic properties of the nanotubes.     

Effects of SWNT and metallic catalyst on hydrogen absorption/desorption performance of MgH2

The microstructure and absorption/desorption characteristics of composite MgH2 and 5 wt % as-prepared single-walled carbon nanotubes (MgH2-5ap) obtained by the mechanical grinding method were investigated. Experimental results show that the MgH2-5ap sample exhibits faster absorption kinetics and relatively lower desorption temperature than pure MgH2 or MgH2-purified single-walled carbon nanotube composite. Storage capacities of 6.0 and 4.2 wt % hydrogen for the MgH2-5ap composite were achieved in 60 min at 423 and 373 K, respectively. Furthermore, its desorption temperature was reduced by 70 K due to the introduction of as-prepared single-walled carbon nanotubes (SWNTs). In addition, the different effects of SWNTs and metallic catalysts contained in the as-prepared SWNTs were also investigated and a hydrogenation mechanism was proposed. It is suggested that metallic particles may be mainly responsible for the improvement of the hydrogen absorption kinetics, and SWNTs for the enhancement of hydrogen absorption capacity of MgH2.